The present invention relates to signal processing apparatus and methods for ameliorating channel effects in a signal received through a propagation channel, e.g. between a node B and a UE in a UMTS network.
High-Speed Downlink Packet Access (HSDPA) is an evolution of the Release 99 version of the 3GPP standard aimed at providing improved user experience through increased data rates and reduced end-to-end latency. These improvements are delivered through a combination of Incremental Redundancy (IR) and the use of higher-order modulation schemes. HSDPA extends the capabilities of 3GPP by introducing the use of the 16QAM modulation for the data bearing channels. 16QAM is more spectrally efficient than the QPSK modulation used in 3G. However, it is also more sensitive to impairments introduced in the transmission link. Hence, in order to fully exploit the benefits of the new features introduced in HSDPA, it is important to select an implementation of the demodulation unit which is resistant to noise and interference.
For W-CDMA systems, it is typical to use a Rake architecture in the receiver (CDMA—Principles of Spread Spectrum Communication, Andrew J. Viterbi, Addison-Wesley Wireless Communications Series). The Rake receiver combines the contributions from the different paths in the propagation channel in order to generate samples to be processed by the channel decoder. The Rake receiver is therefore able to exploit the diversity provided by the propagation channel. However, the decisions generated by the Rake receiver suffer from an increase in noise level due to ISI.
More recently, new receiver architectures have been introduced where the demodulation accuracy is improved at the expense of the implementation complexity. The Linear Minimum Mean Square Error (LMMSE) equaliser is an example of such an architecture (Chip-Level Channel Equalization in WCDMA Downlink, K. Hooli, M. Juntti, M. J. Heikkila, P. Komulainen, M. Latva-aho, J. Lilleberg, EURASIP Journal on Applied Signal Processing, August 2002). The LMMSE equaliser improves the performance of the demodulation unit by mitigating the distortions introduced by the propagation channel. The LMMSE equaliser can be implemented using a pre-filter Rake architecture (Equalization in WCDMA terminals, Kari Hooli, PhD thesis, 2003) where the conventional Rake receiver is preceded by a linear filter which aims at removing the ISI introduced by the channel.
The information intended for a user may be sent over more than one logical channel. In HSDPA for example, the information is sent over the combination of one control and one dedicated channel. The dedicated HS-DSCH channel contains the information intended for a specific user. The HS-SCCH control channel is used to carry information on the format of the HS-DSCH transmission. Both QPSK and 16QAM modulation schemes can be used on the HS-DSCH channel. In order to demodulate the transmitted information, the receiver needs to know which modulation scheme was employed by the transmitter. Information of the modulation scheme is transmitted on the HS-SCCH channel. The HS-SCCH also carries information required to configure the channel decoding section of the receiver. Hence, both the HS-SCCH control channel and the HS-DSCH dedicated channel need to be processed at the receiver in order to recover the transmitted information.
The HS-SCCH channel is also used to control and signal the multiplexing of the different users in the downlink. Users do not necessarily receive information continuously. The set of users to which information is transmitted from the node B can vary at each Transmission Time Interval (TTI) boundary. It is therefore required to indicate to the different users the TTIs during which they should be trying the demodulate data on the HS-DSCH. This is performed using the HS-SCCH control channels. Typically, a receiver will listen to the HS-SCCH so that it can detect when information needs to be received on the HS-DSCH channel. Hence, the receiver typically needs to process the HS-SCCH channel continuously (this depends on the capability class of the receiver) but should only process the HS-DSCH channel when signalled to do so.
The HS-SCCH and HS-DSCH channels are transmitted with different formats. The HS-SCCH channel uses a spreading factor equal to 128 and is always QPSK modulated. The spreading format of the HS-DSCH is lower and equal to 16. Moreover, both QPSK and 16QAM modulations can be used on the HS-DSCH channel.